docs: def create(self, *args, **kwargs):
          """Add a resource to this collection."""
          href = self.url
          if len(args) == 1:
              kwargs[self.model_class.primary_key] = args[0]
              href = '/'.join([href, args[0]])
          model = self.model_class(self,
                                   href=href.replace('classifications/', 'classification/'),
                                   data=kwargs)
          model.create(**kwargs)
          self._models.append(model)
          return model